1. Field of the Invention
The present invention relates to accessory equipment for use on telephone subscriber lines that appear in a telephone central office.
2. Description of the Prior Art
In the operation of commercial telephone systems, subscriber lines are all connected from the different buildings where the subscribers are located to a central testing and access location, termed a telephone central office. At the telephone central office, subscriber lines are terminated and are connected to channels and trunks whenever a call is initiated or received on a subscriber line. The interconnection of the subscriber line with other equipment to complete a telephone call requires the appearance of certain signals on two leads, termed the tip and ring leads, of the subscriber line.
When the subscriber handset is in an idle or "on-hook" condition, a direct current voltage is continuously applied to the tip and ring leads of the telephone subscriber line. This central office subscriber line power is typically applied at a voltage level of about 48 volts, although subscriber line voltages may typically vary between about 48 and about 52 volts, d.c. When a subscriber lifts the receiver of his telephone handset, the voltage level of the central office subscriber line power on that subscriber's line drops sharply, thus initiating seizure of an outgoing channel and trunk in a telephone central office. The seizure of a channel and trunk is necessary for a telephone call to be completed from the subscriber line.
When a telephone subscriber lifts the receiver to an "off-hook" condition, dial tone is produced and a central office channel and trunk are seized. However, not infrequently a subscriber will, for various reasons, never complete a call but simply leave the receiver in an "off-hook" condition. When this occurs, a central office channel and trunk remain connected to that subscriber line until the "off-hook" condition is ultimately terminated.
Studies of subscriber usage have shown that typically no more than about 10% to 20% of subscriber lines that terminate at a telephone central office are in use at any one time. Accordingly, the number of channels and trunks installed at the telephone central office is selected and determined based upon this usage factor.
In recent years, there has been an ever increasing tendency for subscribers to leave their telephone receivers in an "off-hook" condition. This has resulted in an excessive demand for central office channels and trunks. Accordingly, it has been necessary to expend inordinately large amounts of money to install additional trunks and channels in central offices, even though some of these channels and trunks are merely seized and held by telephone subscriber lines which are left in an "off-hook" condition.
Another problem which occurs not infrequently in connection with serving telephone subscriber lines is the problem of unauthorized usage of a subscriber line for a service which the subscriber has declined to accept and for which the subscriber is not charged. Specifically, a subscriber is charged a different rate for the provision of a telephone subscriber line for use with touch-tone, push-button handsets, as contrasted with use of the subscriber line only for the earlier rotary dial or pulse coded type telphone handsets. Nevertheless, even though the customer is paying only for the lower grade service, some subscribers connect telephone touch-tone, push-button handsets to telephone subscriber lines which are only authorized for use with rotary dial telephones. Accordingly, the telphone company is deprived of revenue to which it is rightfully entitled, and the general population of telephone subscribers suffers as a result.